Prevents a WodPress Core Theme from being activated.

gistfile1.phtml

<?php
/*
 * Prevents a WodPress Core Theme from being activated.
 *
 * As a Theme Framework, it provides all the help and functionality child themes need, but child themes
 * should be activated instead.
 *
 * This should go into the Core Theme's functions.php file
 */
if(! class_exists('CORE-THEME-MAIN-CLASS-NAME')) {
    add_action('after_switch_theme', '_core_prevent_activation', 10, 2);
    function _core_prevent_activation($oldThemeName, $oldTheme)
    {
        add_action('admin_notices', '_core_do_not_activate');
        function _core_do_not_activate()
        {
            echo '<div class="update-nag">';
            _e('Theme not activated: the Core Theme is a Theme Framework. Activate the child theme instead!<br/>
                    For now, the previous theme was reactivated.', 'text-domain');
            echo '</div>';
        }

        // Revert to the previous used theme
        switch_theme($oldTheme->stylesheet);
        return false;
    }
}